fix: classify xianyu item auth and blocked states

[jackwener]

Description

This follow-up PR restores the xianyu item auth/risk-control classification fix that did not land in #696 and also updates the test mock to match the current IPage interface so typecheck stays green.

What changed

• classify login walls as AuthRequiredError before waiting on window.lib.mtop
• classify verification/risk-control pages as EmptyResultError
• keep true mtop-not-ready failures as SelectorError
• align the xianyu item test page mock with the current IPage contract

Verification

• npm run test:adapter -- src/clis/xianyu/item.test.ts src/clis/xianyu/search.test.ts src/clis/xianyu/chat.test.ts
• npm run typecheck

Related: #696